Usando este exemplo modificado deos guias do Rails, como modelar uma associação relacional "has_many: through" usando mongoid?

O desafio é que o mongoid não suporta has_many: através do ActiveRecor

# doctor checking out patient
class Physician < ActiveRecord::Base
  has_many :appointments
  has_many :patients, :through => :appointments
  has_many :meeting_notes, :through => :appointments
end

# notes taken during the appointment
class MeetingNote < ActiveRecord::Base
  has_many :appointments
  has_many :patients, :through => :appointments
  has_many :physicians, :through => :appointments
end

# the patient
class Patient < ActiveRecord::Base
  has_many :appointments
  has_many :physicians, :through => :appointments
  has_many :meeting_notes, :through => :appointments
end

# the appointment
class Appointment < ActiveRecord::Base
  belongs_to :physician
  belongs_to :patient
  belongs_to :meeting_note
  # has timestamp attribute
end